Internet protocol (IP) networks are ubiquitous in businesses and enterprises and local area networks (LAN) are commonplace. Expertise with LANs and LAN components is also widespread. Many LAN components are readily available in the market and readily deployable, and various high performance LAN components (e.g., 1 Gb/sec) are becoming increasingly available. Internet Small Computer Systems Interface (iSCSI) is an emerging technology capable of capitalizing on these industry conditions to produce high performance storage networks at a relatively low cost in terms of capital expense and human resources. In general, iSCSI is a transport protocol for establishing and managing connections between IP-based storage devices, hosts, and clients. The iSCSI protocol provides a mechanism for encapsulating SCSI commands for transmission over an IP network.
FIG. 1 illustrates a simple example of the iSCSI protocol in operation. In this example, an iSCSI initiator device 10 is attempting to store data at an iSCSI target device 14 across over a network 18. The initiator 10 sends a protocol data unit (PDU) encapsulating a SCSI write command. In reply, the target 14 queues the command, prepares buffers, and sends a PDU encapsulating a ready-to-transmit (R2T) message to the initiator 10. When the initiator 10 receives the R2T PDU, it begins to transmit a data PDU. When the target 14 receives the data, it buffers the data and replies with another R2T message. The initiator 10 and target 14 may exchange additional data and R2T PDUs until the initiator 10 sends the final SCSI data. The target 14 then replies with a Status PDU indicating that the transfer of the data was successful.
Another emerging trend is the implementation of storage networks over Wide Area Networks (WAN) using long distance communication lines. Long distances between initiator and target devices, however, can result in low performance during storage transmission. The cause is latency: the time required for the commands, data, status, and other iSCSI PDUs to complete a round trip between the initiator and the target. Depending upon the distance and the particular communication paths taken by the iSCSI PDUs through the WAN, latency for a single request and response can be in the order of tens to hundreds of milliseconds. In addition, data transfers can involve several request and response transactions, which operate to magnify the overall latency for a full write transaction. Consequently, such latency can be an impediment to the adoption of iSCSI for storage networks implemented across WANs.